One aspect of the native language sound system that infants must acquire is phonotactic informa- tion, which refers to the constraints on phoneme combinations in a given language as well as the prob- abilities with which phonemes and phoneme combinations occur. A key component of a language’s phonotactic inventory is the distinction between those sound combinations that are attested in the words of the language (i.e., phonotactically legal sequences) and those sound combinations that are unattested (i.e., phonotactically illegal). At a young age, infants distinguish between phonotactically legal and illegal sound sequences. When presented with lists of novel words that consist of phonotac- tically legal or illegal word forms, 9-month-old infants listen longer to the legal word forms. Also at 9 months of age, infants can make an even more fine-grained distinction between two types of word forms that are phonotactically legal—those that consist of high-frequency pho- nemes and phoneme combinations versus those that consist of low-frequency phonemes and pho- neme combinations. Some experiments have demonstrated increasing sensitivity to native lan- guage phonotactics from 6 to 9 months of age, indicating that phonotactic knowl- edge develops over time and with increased linguistic experience.